This invention is directed to pipe supports and pipelines and, in one aspect to a novel support system for thermoplastic piping (e.g. HDPE) which is suitable for use above grade. This system is particularly useful in supporting piping placed in pipe racks or connected to process equipment without causing stress on the equipment or pipe fittings. With such systems design requirements for structural support is reduced.
High density polyethylene (HDPE) and other thermoplastic materials have been formed into piping. Such piping has provided effective solutions to many material handling problems. Typically such piping is used to handle hazardous materials, such as corrosive fluids. Units or lengths of such piping are generally connected with flanges or welding. Overland pipelines often suffer from thermal expansion which causes the pipe to "snake" or "roll" and the pipe fittings to fail at their mitered welds due to over stressing. Thermal expansion in thermoplastic piping is aggravated in above ground piping (i.e. overland pipelines) by the use of plastic additives, such as carbon black, to block the degradation caused by exposure to ultraviolet light. For example, carbon black filled polyethylene piping can expand 0.25 inch (in.) per 100 feet (ft.) of length per each 10.degree. F. rise in temperature. Typically this type of piping is buried underground to control or mitigate the thermal expansion and contraction of pipe due to changes in temperature. The Driscopipe Systems Design (1988), and Engineering Characteristics of Driscopipe, No. 1159-88A17, both by Philips Driscopipe, Inc., discuss methods used in the industry to control the "snaking" or "rolling" caused by thermal expansion. For above ground piping systems that contact the ground, anchoring the pipe at specific intervals or allowing the pipe to move between two rows of pylons anchored in the ground restricts lateral movement. Above ground systems that are supported in pipe racks or suspended in the air present a more difficult problem since the support system must be able to support the weight of the pipe and materials in the pipe. The support system must also withstand the forces exerted by the piping during expansion and contraction.
Continuous support may be used where support structures can handle the stress caused by expansion and contraction of the pipe and the weight of the continuous support, in the form of an external casing or other suitable method. Continuous support may be suitable for piping less than 12 inches in diameter, but the cost of such a structure is prohibitive for large diameter piping. Another prior art method involves insulating the pipe to maintain it at a relatively constant temperature to avoid expansion or contraction due to changes in ambient temperatures. Continuous support and insulation are expensive and result in higher initial costs and in higher maintenance costs if a leak occurs. If a leak occurs, the support or insulation must be removed in order to find and repair the leak. This is difficult, time consuming and costly.
The problems associated with above ground piping along a linear pipe segment are compounded when a system involves interconnected three dimensional piping and tank systems. The continuous support and insulation techniques mentioned above are not suited for use in such a three dimensional situation, particularly when vertical pipe runs are required. The connecting points that join vertical piping to tanks are severely stressed when the pipe expands or contracts.
Various approaches have been used to keep pipe joints from separating due to internal or external pressures. One approach uses threaded pipe sections (U.S. Pat. No. 1,680,499 assigned to S. R. Dresser Manufacturing). Clamping rings in conjunction with a coupling sleeve have been used to reinforce screwed joints. U.S. Pat. No. 1,941,358 (assigned to The Elk River Concrete Products Co.), utilizes clamping rings connected by wire cables spaced around the pipe as a connecting truss to overcome movement caused by shifting soil. Use of support plates, interlocked to keep them from moving relative to each other is taught by U.S. Pat. No. 3,819,210 (assigned to Johns-Manville Corporation) as a means of overcoming internal thrust forces in a pressurized fluid. U.S. Pat. No. 3,252,192 discloses the use of clamps which grip the wall of the pipe, the clamps connected by screw elements, to position and hold adjacent ends of piping together. U.S. Pat. Nos. 3,863,182 (assigned to Owens-Illinois, Inc.), 4,492,391 (assigned to Star Industries, Inc.), 4,602,810 and 4,635,970, each incorporated herein by reference, also disclose systems which utilize thrust rods and couplings to hold pipe sections together.
U.S. Pat. No. 3,930,675 (assigned to Chemiebau Dr. A. Zieren GmbH & Co.) addresses the expansion or contraction of pipe systems by use of expansion compensators to absorb stresses. Use of this method requires that the compensators be compatible with the materials to be carried in the piping and be able to withstand flexing at the bellows without failing or leaking. Metal is usually used in such applications due to its ability to flex without losing the ability to contain the material in the pipeline. Thermoplastic materials ordinarily used in piping do not have the resilience required to flex and maintain structural integrity.
None of the patents cited above, however, satisfactorily address the problems caused by thermal expansion or contraction of thermoplastic piping and there is generally a strong economic and performance preference for above ground thermoplastic piping, particularly when piping corrosive or toxic fluids.